Metals such as chromium are often found in industrial effluents in the toxic hexavalent state and must be removed to eliminate possible health and environmental hazards. Examples of aqueous industrial solutions from which hexavalent chromium must be removed are industrial waste waters, rinse waters, chromium plating baths, etching solutions and electrochemical machining solutions.
In particular, industrial processes such as electrochemical machining of stainless steel alloys utilize aqueous electrolyte solutions such as sodium nitrate or sodium chloride. During the process at least part of the chromium contained in the alloy is converted to hexavalent chromium in the electrolyte solution where it remains in solution over a wide range of pH. Because the electrolyte is continuously reused, the accumulation of hexavalent chromium in solution represents a possible health hazard to machine operators. Also, most of the machined metal is converted to an oxide/hydroxide sludge which entrains some electrolyte and poses an environmental hazard. The sludge is thus disposed of as a hazardous waste at considerable expense. It is therefore desirable to diminish the chromium concentration in the electrolyte to as low a level as possible while still maintaining the properties of the aqueous electrolyte.
One common method for removing chromium from aqueous solutions involves the reduction of hexavalent chromium to trivalent chromium using sulfur dioxide or soluble sulfite-containing compounds such as sodium bisulfite, metabisulfite, or hydrosulfite after adjusting the pH to about 2-3 with sulfuric acid. The trivalent chromium produced is then precipitated from solution with a base such as sodium hydroxide after the solution pH has been adjusted to make it alkaline. The precipitated chromium is then separated from the solution using common techniques such as settling or filtration, for example. However, use of this method for removal of chromium from electrolyte solutions is undesirable because further treatment is necessary to remove sulfur compounds from the solution.
Another well-known method for removing chromium from aqueous solutions involves ferrous ion reduction of hexavalent chromium to trivalent chromium and subsequent precipitation of the trivalent chromium after adjusting the pH. Typically, ferrous sulfate (FeSO.sub.4) or ferrous chloride (FeCl.sub.2) is used as the reducing agent, and chromic hydroxides and iron hydroxides are formed and precipitated under alkaline conditions. However, the sulfate or chloride remains in solution and may require further removal treatment. A procedure to remove the dissolved sulfate from a contaminated sodium nitrate solution, for example, is disclosed in copending commonly owned Application Ser. No. 08/080549 where barium nitrate is added to the solution to induce precipitation of the sulfate ion. Sodium hydroxide is then added to complete the precipitation of the iron and chromium species. Alternatively, barium hydroxide may be used instead of barium nitrate, and the addition of nitric acid is required to neutralize the sodium hydroxide formed.
Alternatively, ferrous ions can be introduced into solution by electrochemical generation. A chromium-contaminated aqueous solution is passed between cold-rolled steel electrodes, and the direct current between the electrodes forms ferrous ions (Fe.sup.+2) at the anode and hydrogen gas and hydroxide at the cathode. Hexavalent chromium is reduced by the ferrous ion to trivalent chromium, and the ferrous ion is oxidized to the ferric ion (Fe.sup.+3). For every mole of hexavalent chromium reduced, three moles of ferrous iron must be oxidized. Trivalent chromium, ferric ions, and leftover ferrous ions remaining in solution react with hydroxide formed at the cathode to form insoluble chromium and iron hydroxides. A polymer is then added to the solution to accelerate precipitation of the hydroxides which can subsequently be filtered from the solution. However, the method is disadvantageous for removing chromium from aqueous solutions because expensive and complex equipment must be utilized to generate the ferrous ions.
To eliminate the necessary step of sulfate precipitation and to avoid the costly and complex process of ferrous ion generation, the development of an inexpensive and efficient method of ferrous reduction which introduces no new ions to the electrolyte solution is desirable. For example, sodium nitrate is a common electroyte used in electrochemical machining, and a ferrous reducing agent that adds only nitrates or hydroxides to solution is preferable for the removal of chromium. However, ferrous nitrate is not commercially available and its use would thus be impractical.